The etiology of clinically isolated, severe, pure mitral regurgitation was determined in 97 patients over 30 years of age by examination of the operatively excised mitral valves. None had any degree of mitral stenosis or a dysfunctioning aortic valve as determined by preoperative catheterization. The etiology of the mitral regurgitation was leaflet prolapse in 60 patients (62%), papillary muscle dysfunction from coronary heart disease in 29 (30%), infective endocarditis on previously normally functioning valves in 5 (5%), and rheumatic heart disease in 3 (3%).